Olefin polymerization systems are divided into Ziegler-Natta and metallocene catalyst systems, and these two catalyst systems have been developed corresponding to their respective characteristics.
The Ziegler Natta catalyst has been widely applied in existing commercial processes since it was invented in the fifties, but since it is a multi-site catalyst with several active sites, it is characterized by a wide molecular weight distribution of a polymer, and is limited in terms of securing of desired properties due to non-uniform composition distribution of comonomers. Particularly, since a propylene polymer prepared using the Ziegler-Natta catalyst has a high xylene soluble content (for example, greater than 5 wt %), when a Ziegler-Natta catalyst is used, it is difficult to obtain a propylene polymer with a low melting point (Tm).
Meanwhile, the metallocene catalyst consists of a main catalyst, of which a main component is a transition metal compound, and a cocatalyst, which is an organometal compound including aluminum as the main component, and such a catalyst is a homogeneous complex catalyst and a single site catalyst. Thus, the metallocene catalyst enables formation of a polymer with a narrow molecular weight and a uniform comonomer composition distribution. Further, the metallocene catalyst can change the stereoregularity, copolymerization property, molecular weight, crystallinity, etc. according to modification of the ligand structure of the catalyst and change of polymerization conditions.
U.S. Pat. No. 5,032,562 (1991 Jul. 16) describes a method for preparing a polymerization catalyst by supporting two different transition metal catalysts on one carrier. This is a method of producing a bimodal distribution polymer by supporting a titanium (Ti)-based Ziegler-Natta catalyst producing high molecular weight and a zirconium (Zr)-based metallocene catalyst producing low molecular weight on one carrier, and has disadvantages in that the supporting process is complicated, and polymer morphology is deteriorated due to the cocatalyst.
U.S. Pat. No. 5,525,678 (1996 Jun. 11) describes a method of using a catalyst system for olefin polymerization that simultaneously supports a metallocene compound and a non-metallocene compound on a carrier, thus simultaneously polymerizing a high molecular weight polymer and a low molecular weight polymer. It has disadvantages in that the metallocene compound and the non-metallocene compound should be separately supported, and the carrier should be pretreated with many compounds for supporting.
U.S. Pat. No. 5,914,289 (1996 Jun. 22) describes a method of controlling the molecular weight and molecular weight distribution of a polymer using metallocene catalysts supported on each carrier, but a large amount of solvent is required when preparing the supported catalyst and a long preparation time is required, and the used metallocene catalysts should be respectively supported on a carrier, which is inconvenient.
Korean Patent Application No. 10-2003-0012308 (2003 Feb. 27) discloses a method of controlling molecular weight distribution by supporting a binuclear metallocene catalyst and a mononuclear metallocene catalyst on a carrier together with an activator, and polymerizing while changing the combinations of catalysts in the reactor. However, this method is limited in terms of the simultaneous realization of the properties of each catalyst, and has disadvantages in that the metallocene catalyst part is isolated from the carrier component of the finished catalyst to induce fouling in the reactor.
Therefore, in order to solve the above problems, there is a continued demand for a method for preparing a supported metallocene catalyst with excellent activity in a convenient way to prepare a polyolefin with desired properties.